1. Field of the Invention
The present invention relates to an ultrasonic sensor which is used for a vehicle and the like.
2. Description of Related Art
An ultrasonic sensor is used as an obstacle detection sensor for detecting obstacle by transmitting and receiving an ultrasonic wave. This ultrasonic sensor has a piezoelectric vibrator. The piezoelectric vibrator vibrates to transmit an ultrasonic wave and receives a reflected wave from an obstacle, so that an obstacle can be detected.
However, the above ultrasonic sensor has a disadvantage that a reverberation occurs after normal vibration of the piezoelectric vibrator. Therefore, detection accuracy of the sensor may be reduced and the ultrasonic sensor may not be able to detect an obstacle at a short distance if the reverberation continues long.
In order to adjust the reverberation period to be within a suitable range, a capacitor to compensate for an electric capacity of the piezoelectric vibrator can be used.
In JP-A-H8-237796, as shown in FIG. 8, an ultrasonic sensor has a capacitor J2 connected in parallel with a piezoelectric vibrator J1 in order to reduce a temperature drift of a sensor output. When a temperature of the ultrasonic sensor changes, an electric capacity of the capacitor J2 changes in the opposite direction of an electric capacity change of the piezoelectric vibrator J1. That is, the electric capacity change of the piezoelectric vibrator J1 can be cancelled.
However, characteristics of the piezoelectric vibrator J1 and the capacitor J2 are uneven. Therefore, an increase amount of electric capacity of the piezoelectric vibrator J1 and a decrease amount of electric capacity of the capacitor J2 are not equal. Accordingly, a temperature compensation cannot be performed correctly and a temperature drift occurs. Further, a temperature range to compensate for electric capacity may be limited according to characteristics of the capacitor J2. Therefore, the technique disclosed in JP-A-H8-237796 is not satisfactory to adjust a reverberation period.
In JP-A-H11-103496, an ultrasonic sensor has a capacitor alternately connected and disconnected in parallel with a piezoelectric vibrator in order to improve a sound pressure of an ultrasonic wave to be transmitted and a sensitivity to a reflected wave. The capacitor is connected when the piezoelectric vibrator transmits an ultrasonic wave. To the contrary, the capacitor is disconnected when the piezoelectric vibrator receives a reflected wave. Thus, a resonance frequency and an anti-resonance frequency are changed to improve a sound pressure of an ultrasonic wave to be transmitted and a sensitivity to a reflected wave. However, a reverberation period cannot be suitably adjusted only by connecting or disconnecting a capacitor. Therefore, the technique disclosed in JP-A-H11-103496 is not satisfactory to adjust a reverberation period.